1. Field of the Invention
The present invention relates to a method for estimating a residual capacity of a secondary battery.
2. Description of the Prior Art
In the case where a secondary battery is used in an electric vehicle, for example, estimating a residual capacity of the secondary battery is important to know the mileage possible to gain before the next charge. Also, in the case where a secondary battery is used in uninterruptible power supply equipment, it is important to know a backup time at a power failure.
Consequently, methods for estimating a residual capacity of a secondary battery have been proposed. Estimating a residual capacity by measuring an open circuit voltage of a storage battery is one of these methods. This method is based on such relationships that an open circuit voltage of a lead acid battery is proportional to a sulfuric acid concentration in an electrolyte, whereas the sulfuric acid concentration is correlated with a residual capacity. However, this method has a problem that a water loss during the use of the battery would cause an increase of the sulfuric acid concentration and would change the relationship between the sulfuric acid concentration and the residual capacity, which would result in the lower estimating accuracy.
On the other hand, there also is an attempt to determine a residual capacity by integrating the electrical charge-discharge quantities of a secondary battery in use. For example, in the method disclosed in Japanese Patent Application Laid-Open No. H8-240647, a residual capacity (SOC: State Of Charge) was calculated according to an approximate linear function previously determined by measuring a discharge current and a discharge voltage from a battery while electricity was being discharged from the battery. Further, in the method disclosed in Japanese Patent Application Laid-Open No. H11-38107, a charge-discharge current and temperature were measured first, and then, SOC was calculated by integrating the electrical charge-discharge quantities while correcting them depending on the temperature.
However, above-described methods have a problem that when the battery has been used over a prolonged period, cumulative error would gradually lower the accuracy. To avoid the cumulative error, the integrated value has to be reset when the battery is fully charged. However, in the case where the battery is continuously used in partially charged state, the cumulative error is inevitable because there is no chance to reset the value.
Moreover, a lead acid battery, for instance, will be deteriorated because of various factors like self-discharge, grid corrosion, water loss, softening of a positive active material, lead sulfate accumulation onto a negative active material, etc. All the above-described methods have a problem that the estimating accuracy would become poorer after the deterioration.
Therefore, an object of the present invention is to provide a method for estimating a residual capacity of a secondary battery with high accuracy even after a deterioration of the secondary battery.